The biggest political risk is the potential that the US result won‘t be known due to the combination of a tight election and re-count/mail-in issues in swing states.

Citi's Steven Englander:

If the election is uncertain – which is possible if voting is very close in key states – it would probably be viewed as raising the odds of a Romney win, or at least raising uncertainty. On that basis, we would lean to initial concern that we would see austerity both on the fiscal and monetary side – commendable from an ideological viewpoint, but unlikely to inspire confidence in asset markets. So we would lean to partial risk-off.

The most interesting comment comes from SocGen's Kit Juckes, who brings the view from Europe:

Vote early, often and well. That's about all we an ask of Americans. I cannot see an outcome that would make me optimistic about growth in Europe, or keen to own the euro. The best the US can do for the rest of us is to tur the Fiscal Cliff into something more like a fiscal pimple, and re-become a driver of global growth, 'cos we ain't doing nothing this side of the Atlantic to help ourselves.